The present invention relates to a method of image density adjustment which can change the tone of a halftoned image by a dither method or an error diffusion method employed in an image processing apparatus such as a personal computer, a scanner, a printer, a facsimile telegraph and a digital copier, without deteriorating resolution of the image, and to an apparatus using the method of image density adjustment.
In an the image processing apparatus such as a personal computer, a scanner, a printer, a facsimile telegraph and a digital copier, halftone images by a dither method or an error diffusion method are widely used. These methods can keep satisfactory gradation and resolution of an image while they reduce data sizes of the image.
However, binary halftone images represent a congregation of on/off data and they are difficult to be changed freely in terms of tone (.gamma.) as they are. In this respect, they are different from multi-level images. Solution of this technical problem requires processing wherein an original multi-level image is estimated from a binary halftone image, tone (.gamma.) of the estimated multi-level image is changed, and a dither method or an error diffusion method is used again for halftoning. Each of TOKKAISHO Nos. 63-234672 and 63-290072 discloses a technology to estimate an original multi-level image from a binary halftone image, In this technology, an N.times.N sized window is provided of which a target pixel is in the center, and the number of black pixels counted in that window is weighted to estimate a value of the target pixel.
However, in the method disclosed in the publications of TOKKAISHO Nos. 63-234672 and 63-290072, there is a possibility of deterioration of resolution because information of plural pixels are averaged. For example, fine lines having a width which is the same as that of a pixel turn into thick and light lines, or small letters turn into blurred letters which are not legible. When the multi-level image thus estimated is multiplied by .gamma. to be changed in terms of tone, and then is binary halftoned again, there is a possibility that fine lines and image quality reproduced on a binary image are deteriorated.
In view of the technical problems stated above, an object of the invention is to provide a method of image density adjustment and an apparatus using the same, which can change the tone of an image which is halftoned to a binary-level or multi-level by a dither method or an error diffusion method, without deteriorating resolution of the image.
The object mentioned above can be attained by either one of the following structure.
A method of image density adjustment wherein n-bit image signals are inputted, a first gray-level number converting processing is conducted to change image signals of n-bit to those of m-bit that is greater than n-bit, and to change image signals of m-bit to those of t-bit that is greater than m-bit, and a the second gray-level number converting processing is conducted to change the image signals of t-bit to those of s-bit that is smaller than t-bit.
The methods mentioned above can control, without deteriorating resolution and gradation, .gamma. (brightness) of the image which is halftoned to binary-level or multi-level by an error diffusion method, a dither method or a density pattern method so that gradation may be represented with plural pixels.